A blood sugar level is regulated by glucose uptake in the skeletal muscle and adipose tissue through the action of insulin. In diabetes mellitus, this lowered action of insulin results in maintaining a high blood sugar level, which leads to the development of diabetes mellitus. Uptake of glucose in cells requires to be mediated by a membrane protein called a glucose transporter. Currently known glucose transporters are 7 types of GLUT1 through GLUT7 (Bell et al., J. Biol. Chem., 268, 3352–3356, 1993; Olson & Pessin, Annu. Rev. Nutr., 16, 235–256, 1996). Among them, glucose transporter 4 (GLUT4), which expression is noted chiefly in the skeletal muscle and adipose tissues, is associated with the insulin-induced glucose transporting activity (Fukumoto et al., Proc. Natl. Acad. Sci. USA., 85, 5434–5438, 1988; Birnbaum et al., Cell, 57, 305–315, 1989).
GLUT4 is present normally in the intracellular vesicles called GLUT4 vesicles. When blood sugar increases, it is considered that GLUT4 would migrate into cell membranes (translocation) by the action of insulin to promote the glucose uptake (Bell et al., Diabetes Care, 13, 198–208, 1990; Czech et al., Trend. Biochem. Sci., 17, 197–201, 1992).
In order to clarify the molecular mechanism for this translocation of GLUT4 vesicles, it has been attempted to identify not only GLUT4 itself but also other proteins constituting the GLUT4 vesicles. Presently, IRAP (insulin-responsive aminopeptidase; Kandror & Pilch, Proc. Natl. Acad. Sci. USA, 91, 8017–8021, 1994, Kandror et al., J. Biol. Chem. 269, 30777–30780, 1994, Keller et al., J. Biol. Chem., 270, 23612–23618, 1995) is known as the molecule for constituting GLUT4 vesicles, in addition to VAMPs (vesicle-associated membrane proteins; Cain et al., J. Biol. Chem., 267, 11681–11634, 1992), SCAMPs (secretory component-associated membrane proteins; Thoidis et al., J. Biol. Chem., 268, 11691–11696, 1993; Laurie et al., J. Biol. chem., 268, 19110–19117, 1993), phosphatidylinositol 4-kinase (Del Vacchio & Pilch, J. Biol. Chem., 266, 13278–13283, 1991), low molecular weight GTP-bound protein Rab4 (Cormont et al., J. Biol. Chem., 268, 19491–19497, 1993), etc.
IRAP, also called gp160, is a membrane protein of one transmembrane type and localized in the GLUT4 vesicles in cells. In view of the protein structure, IRAP consists of an amino-terminal (N-terminal) 109 amino acid domain in the cytoplasm, followed by a 22 amino acid transmembrane domain and further an extracellular domain composed of a carboxy-terminal (C-terminal) 785 amino acids (Kandror & Pilch, Proc. Natl. Acad. Sci. USA, 91, 8017–8021, 1994; Keller et al., J. Biol. Chem., 270, 23612–23618, 1995). The extracellular domain is a zinc-dependent protease (aminopeptidase) and its activity has been verified (Kandror et al., J. Biol. Chem., 269, 30777–30780, 1994). A peptide corresponding to the N-terminal side domain (cytoplasmic side domain) among these domains is injected into cells to cause translocation of GLUT4 vesicles to the cell surface. It is thus predicted that an IRAP binding protein will be present to retain GLUT4 vesicles within cells (Waters et al., J. Biol. Chem., 272, 23323–23327, 1997).
The cDNA of FHOS gene and the putative amino acid sequence are also known (Westendorf et al., Gene, 232, 173–182, 1999; Genbank Accession No. AF113615). However, it has not been reported that FHOS binds to the molecules constituting GLUT4 vesicles, such as IRAP, etc.